A slider designed to appear outside of the strike zone that crosses over the back corner of the plate just before it is caught.
The slider is one of the most common lateral-breaking pitches, and can be especially deadly if it can be thrown for strikes. The backdoor breaking ball, which includes both sliders and curveballs is meant to fool the hitter into complacency be appearing as a ball rather than a strike. At the last second, the ball snaps back toward the plate and catches the back corner for a strike, coming into the “back door” of the zone. Backdoor sliders are often favored over curveballs because the speed of the pitch gives the hitter less time to make the decision to swing or hold up. The sharper downward movement of the slider also results in less “hanging” pitches that allow hitters to do a lot of damage from the box.